bcachefs: Fix bch2_btree_path_traverse_all()

We need to take a ref on a path while we're traversing it: this fixes a
bug with paths getting reused while being traversed, in the key cache
fill code.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -999,7 +999,10 @@ static int bch2_btree_path_traverse_all(struct btree_trans *trans)
 		 * the same position:
 		 */
 		if (path->uptodate) {
+			__btree_path_get(path, false);
 			ret = btree_path_traverse_one(trans, path, 0, _THIS_IP_);
+			__btree_path_put(path, false);
+
 			if (bch2_err_matches(ret, BCH_ERR_transaction_restart) ||
 			    ret == -ENOMEM)
 				goto retry_all;